Staked fasteners are used in numerous applications, such as securing multiple pieces of sheet material and mounting devices on boards. Common staked fasteners include rivets and posts.
All staked fasteners function by having a clinch portion that is staked in a staking operation. In the case of a typical rivet, the rivet prior to the staking operation generally has a head and a shank having a grip portion and a clinch portion. In an application where a material combination, e.g., two pieces of sheet material, is being secured together, the rivet is inserted into aligned holes, one in each piece of sheet material, such that the head is on one side of the material combination, the grip portion is in the holes, and the clinch portion is projecting outwardly from the other side of the material combination. The clinch portion is then staked in a staking operation to form a head, thereby trapping the material combination between the two heads.
Staking of the clinch portion can be accomplished using numerous methods, and the choice of method depends on the material from which the clinch portion of the rivet is made. That said, the quality of the staking operation determines the quality of the head formed thereby. As the staking operation defines the quality of the head, it also defines the final mechanical properties of the completed staked fastener.
Ideally, in a staking operation, the clinch portion is shaped into a head such that loads acting on the created head are properly transferred into the grip portion. For example, when a circular head is created on a circular grip portion, it is important that the centerlines of each align. It should be appreciated that misalignment would tend to permit a torque couple that could cause a failure of the staked fastener.
Where staking of several staked fasteners is performed employing hand-held tools, variations in the quality of the formed heads are particularly problematic. When a hand-held tool is used, it is the skill of the operator of the tool that determines the quality of the head. This is particularly true where the staked fastener, or at least the clinch portion, is made from a plastic and the staking method employed involves melting the plastic, such as when the staking operation employs ultrasonics. When melting of the staked portion is employed, the operator of a hand-held tool must assure that during setup of the tool, the tool is properly centered on the clinch portion, and then during the staking operation the tool is forced down along the axis of the clinch portion.
As anyone who has ever worked with a process involving the shaping of plastic by melting with hand tools will attest, there is a significant amount of skill required to achieve a proper result. The process is at best problematic when the plastic is being formed on a horizontal, flat surface, and gravity is acting in the appropriate direction. The complexity only increases when any of these conditions is not present. However, and almost more importantly, a high degree of repeatability or uniformity regardless of conditions across numerous units is all but impossible.
In the case of a staked fastener, this process is further complicated where the staked fastener employs a supplemental material. Supplemental material is a second material applied around the clinch portion of the fastener prior to performing the staking operation, and it becomes part of the fastener. In some cases, the supplemental material will also be melted during the staking operation, which increases the skill required of a hand tool operator to make repeatable, high-quality fasteners.
What is needed in the art is a better way of employing hand tools to stake a staked fastener. More specifically, the art needs a better way of staking a staked fastener where the staking operation involves flowing, e.g., melting, of the clinch portion to increase the consistency of the staking result across multiple staking operations.